ἐπερίσσευσεν - Why is the augment before the preposition?

[ἑκηβόλος]

Why is the syllabic augment of ἐπερίσσευσεν the aorist indicative of περισσεύειν placed before the preposition?

Arising from reading:

Εἰ γὰρ ἡ ἀλήθεια τοῦ θεοῦ ἐν τῷ ἐμῷ ψεύσματι ἐπερίσσευσεν εἰς τὴν δόξαν αὐτοῦ, τί ἔτι κἀγὼ ὡς ἁμαρτωλὸς κρίνομαι;

[Barry Hofstetter]

It was not perceived to be a compound:

περισσεύω, Att. περιττεύω, impf. ἐπερίσσευον (περιέσσευον is condemned by Phryn.20), (περισσός)

Liddell, H. G., Scott, R., Jones, H. S., & McKenzie, R. (1996). A Greek-English lexicon (p. 1387). Oxford: Clarendon Press.

If condemned by the Atticists, that means somebody was seeing it as the equivalent of περι + σσευω, apparently as you are thinking of it, and Phrynichus was saying, "no, not the case."

However, even with prepositional compounds, later Greek sometimes augments before the preposition or even does a double augment. This is frequent enough in the LXX to be annoying.

[ἑκηβόλος]

None of the major early NT manuscripts use the condemned περιέσσευον form in any in any of Jn.6:13, Ac.16:5, Rm.3:7, 5:15, 2Co.8:2, or Eph.1:8.

Is palatal σσ in this word the same as the palatal in the ξ of πέριξ?

[Barry Hofstetter]

None of the major early NT manuscripts use the condemned περιέσσευον form in any in any of Jn.6:13, Ac.16:5, Rm.3:7, 5:15, 2Co.8:2, or Eph.1:8.

Is palatal σσ in this word the same as the palatal in the ξ of πέριξ?

Right, nobody said that anybody used the "condemned form" in any NT text, the NT is using the standard Koine form. -ττ- is simply Attic for -σσ-.